.Dd July 17, 2014
.Dt BIO_F_MD 3
.Os
.Sh NAME
.Nm BIO_f_md ,
.Nm BIO_set_md ,
.Nm BIO_get_md ,
.Nm BIO_get_md_ctx
.Nd message digest BIO filter
.Sh SYNOPSIS
.In openssl/bio.h
.In openssl/evp.h
.Ft BIO_METHOD *
.Fo BIO_f_md
.Fa void
.Fc
.Ft int
.Fo BIO_set_md
.Fa "BIO *b"
.Fa "EVP_MD *md"
.Fc
.Ft int
.Fo BIO_get_md
.Fa "BIO *b"
.Fa "EVP_MD **mdp"
.Fc
.Ft int
.Fo BIO_get_md_ctx
.Fa "BIO *b"
.Fa "EVP_MD_CTX **mdcp"
.Fc
.Sh DESCRIPTION
.Fn BIO_f_md
returns the message digest BIO method.
This is a filter BIO that digests any data passed through it.
It is a BIO wrapper for the digest routines
.Fn EVP_DigestInit ,
.Fn EVP_DigestUpdate ,
and
.Fn EVP_DigestFinal .
.Pp
Any data written or read through a digest BIO using
.Xr BIO_read 3
and
.Xr BIO_write 3
is digested.
.Pp
.Xr BIO_gets 3 ,
if its
.Sy size
parameter is large enough,
finishes the digest calculation and returns the digest value.
.Xr BIO_puts 3
is
not supported.
.Pp
.Xr BIO_reset 3
reinitialises a digest BIO.
.Pp
.Fn BIO_set_md
sets the message digest of BIO
.Fa b
to
.Fa md :
this must be called to initialize a digest BIO
before any data is passed through it.
It is a
.Xr BIO_ctrl 3
macro.
.Pp
.Fn BIO_get_md
places the a pointer to the digest BIOs digest method in
.Fa mdp .
It is a
.Xr BIO_ctrl 3
macro.
.Pp
.Fn BIO_get_md_ctx
returns the digest BIOs context in
.Fa mdcp .
.Sh NOTES
The context returned by
.Fn BIO_get_md_ctx
can be used in calls to
.Xr EVP_DigestFinal 3
and also in the signature routines
.Xr EVP_SignFinal 3
and
.Xr EVP_VerifyFinal 3 .
.Pp
The context returned by
.Fn BIO_get_md_ctx
is an internal context structure.
Changes made to this context will affect the digest BIO itself, and
the context pointer will become invalid when the digest BIO is freed.
.Pp
After the digest has been retrieved from a digest BIO,
it must be reinitialized by calling
.Xr BIO_reset 3
or
.Fn BIO_set_md
before any more data is passed through it.
.Pp
If an application needs to call
.Xr BIO_gets 3
or
.Xr BIO_puts 3
through a chain containing digest BIOs,
then this can be done by prepending a buffering BIO.
.Pp
Before OpenSSL 1.0.0 the call to
.Fn BIO_get_md_ctx
would only work if the BIO had been initialized for example by calling
.Fn BIO_set_md .
In OpenSSL 1.0.0 and later the context is always returned
and the BIO is state is set to initialized.
This allows applications to initialize the context externally
if the standard calls such as
.Fn BIO_set_md
are not sufficiently flexible.
.Sh RETURN VALUES
.Fn BIO_f_md
returns the digest BIO method.
.Pp
.Fn BIO_set_md ,
.Fn BIO_get_md ,
and
.Fn BIO_get_md_ctx
return 1 for success and 0 for failure.
.Sh EXAMPLES
The following example creates a BIO chain containing an SHA1 and MD5
digest BIO and passes the string "Hello World" through it.
Error checking has been omitted for clarity.
.Bd -literal -offset 2n
BIO *bio, *mdtmp;
const char message[] = "Hello World";
bio = BIO_new(BIO_s_null());
mdtmp = BIO_new(BIO_f_md());
BIO_set_md(mdtmp, EVP_sha1());
/*
 * For BIO_push() we want to append the sink BIO
 * and keep a note of the start of the chain.
 */
bio = BIO_push(mdtmp, bio);
mdtmp = BIO_new(BIO_f_md());
BIO_set_md(mdtmp, EVP_md5());
bio = BIO_push(mdtmp, bio);
/* Note: mdtmp can now be discarded */
BIO_write(bio, message, strlen(message));
.Ed
.Pp
The next example digests data by reading through a chain instead:
.Bd -literal -offset 2n
BIO *bio, *mdtmp;
char buf[1024];
int rdlen;

bio = BIO_new_file(file, "rb");
mdtmp = BIO_new(BIO_f_md());
BIO_set_md(mdtmp, EVP_sha1());
bio = BIO_push(mdtmp, bio);
mdtmp = BIO_new(BIO_f_md());
BIO_set_md(mdtmp, EVP_md5());
bio = BIO_push(mdtmp, bio);
do {
	rdlen = BIO_read(bio, buf, sizeof(buf));
	/* Might want to do something with the data here */
} while (rdlen > 0);
.Ed
.Pp
This next example retrieves the message digests from a BIO chain
and outputs them.
This could be used with the examples above.
.Bd -literal -offset 2n
BIO *mdtmp;
unsigned char mdbuf[EVP_MAX_MD_SIZE];
int mdlen;
int i;

mdtmp = bio;	/* Assume bio has previously been set up */
do {
	EVP_MD *md;
	mdtmp = BIO_find_type(mdtmp, BIO_TYPE_MD);
	if (!mdtmp)
		break;
	BIO_get_md(mdtmp, &md);
	printf("%s digest", OBJ_nid2sn(EVP_MD_type(md)));
	mdlen = BIO_gets(mdtmp, mdbuf, EVP_MAX_MD_SIZE);
	for(i = 0; i < mdlen; i++)
		printf(":%02X", mdbuf[i]);
	printf("\en");
	mdtmp = BIO_next(mdtmp);
} while(mdtmp);
BIO_free_all(bio);
.Ed
.Sh BUGS
The lack of support for
.Xr BIO_puts 3
and the non standard behaviour of
.Xr BIO_gets 3
could be regarded as anomalous.
It could be argued that
.Xr BIO_gets 3
and
.Xr BIO_puts 3
should be passed to the next BIO in the chain and digest the data
passed through and that digests should be retrieved using a separate
.Xr BIO_ctrl 3
call.
